It is known that the ultraviolet rays cause various adverse effects on the skin. The ultraviolet rays are classified into UV-A waves having 320 nm to 400 nm, UV-B waves of 290 nm to 320 nm and UV-C waves of not more than 290 nm according to the wavelengths. The UV-C waves are absorbed in the ozone layer, and almost all of them hardly reach the earth. When not less than a given amount of light of the UV-B waves reaching the earth is irradiated upon the skin, it forms an erythematous patch or a blister, so that the formation of melamine is promoted. Further, it is considered that the UV-A waves actually darken the skins, although they are less likely to cause the erythematous patch, as compared with the UV-B waves. Further, the UV-A waves have high permeability to the skins, promote the formation of crosslinking of collagen as a protein in the skins, lower the elasticity and water sustainability of collagen, promote the formation of wrinkles, and cause stains, freckles and skin aging. In addition, since the UV-A waves increase lipid peroxides of skin tissues, it is known that they cause skin cancers.
In order to protect the skins from damages with such ultraviolet rays, cosmetics containing various kinds of ultraviolet shielding agents have been heretofore developed and commercially available. As the various kinds of the ultraviolet shielding agents, there are organic ultraviolet absorbing agents such as benzophenones, aminobenzoic acids, cinnamic acid esters, benzotriazoles, salicylic acids and the like, and inorganic pigments such as titanium dioxide, zinc oxide, zirconia oxide, and iron oxides and the like.
Among them, the inorganic pigments having the ultraviolet shielding effect to be mixed into cosmetics, for example, titanium dioxide, are set to particle diameters in the Rayleigh scattering range, which is smaller than the wavelengths of the visible light. As the set powder particles are more dispersed in a state nearer to primary particle diameters, the ultraviolet ray scattering can be exhibited to the maximum. However, the smaller the primary particle diameters of the powder are, the greater is the specific surface area, so that contact points among the particles increase therefore the powder is more likely to be flocculated, and consequently the dispersability is deteriorated. Many techniques have been proposed as measures to avoid such flocculation as much as possible and prevent re-flocculation even with the lapse of time.
A suntan composition in which microparticulate titanium dioxide and microparticulate zinc oxide are mixed is proposed, for example, but a high SPF value is not obtained unless it is mixed at a high concentration (13 wt % or more). Further, high-concentration mixing is unfavorable in that it increases a hiding power and retains whiteness in a cosmetic film, and the particles are flocculated again with the lapse of time, so that the SPF value not only decreases but also a cosmetic oil agent is denatured, and feeling is deteriorated. (Patent Document 1)
Furthermore, cosmetics containing microparticulate titanium dioxide treated with silicone, microp articulate zinc oxide treated with silicone and a clay mineral treated with silicone are proposed. This can offer cosmetics having high stability free from change with the lapse of time, which have good dispersability and a high UV shielding effect, suppress optical activity and catalytic activity of an ultraviolet protecting material. Furthermore, regarding the lasting cosmetic quality, excess secretion of sebum causes make-up deterioration such as <unevenness>, <oily sheen>, <creasing>, <skin dullness>, <disappearance> and the like owing to change in a cosmetic film applied onto a skin with lapse of time, and further causes the production of a peroxide with oxygen in the atmosphere. When such stimulants enter the skin, they cause inflammation or abnormal cuticle, or results in spots or freckles like optically deteriorated fat and oil. Therefore, the cause of the above peroxide is prevented by blending zinc oxide, adsorbing and solidifying as an effective treatment of the skin under consideration of skin safety. However, safety of zinc oxide has been recently questioned, and some limitation is posed on uses thereof. (Patent Document 2)
As to the lasting cosmetic quality, a powder having a layer of hydroxy apatite and a layer of zinc oxide layer and a powder in which sericite particles are coated with amorphous calcium phosphate are proposed to provide cosmetics having excellent feeling, while maintaining a function of adsorbing and removing an excreted waste product. However, although an effect of the lasting cosmetic quality can be expected, no consideration is made of enhancement of the dispersibility among fine particles exhibiting an UV shielding effect or the prevention of re-flocculation with the lapse of time. In addition, the production costs of these powders are high. (Patent Documents 3 and 4)
Furthermore, in order to improve the lasting cosmetic quality and enhance water repellency and oil repellency against sweat and sebum, cosmetics containing fluorine-treated powders are proposed. Although the cosmetics using the powder are less wetted with the skin and sweat, an attachment amount to the skin is small because the water repellency and the oil repellency are high. Consequently, a phenomenon that the powder slides on the skin to <twist> a cosmetic film is seen, so that a make-up effect is damaged. Moreover, fluorine compounds such as a perfluoroalkyl phosphoric acid ester diethanolamine salt and the like contain a diethanolamine which is unfavorable from a standpoint of safety. There is a problem that producing steps becomes complicated so as to remove this. (Patent Document 5)